‘MODEL’ MARRIAGE GONE BAD: Robb, here with supermodel then-wife Veronica Webb, cheated his business partner and siblings out of millions of dollars, according to two federal lawsuits.Greg Kessler/Patrick McMullan

A Wall Street scion once married to supermodel Veronica Webb is no friend — and a lousy brother, two blockbuster federal lawsuits claim.

George Robb Jr. and his supermodel ex-wife have been inhabiting a pair of swanky penthouses that were supposed to be sold for millions, according to one suit, filed by his former business partner and best friend.

Robb also faces a suit from four siblings who say he cheated them out of their fair share of the $180 million “windfall” he reaped by selling their late father’s trading firm.

Low, a veteran real-estate developer who ran a subsidiary of Robb’s company, claims he and Robb bought two downtown properties with plans to turn them into luxury apartments and sell them for a profit.

But when Robb’s then-wife, Webb — a former Revlon spokesmodel and Victoria’s Secret stunner — got pregnant in 2001, the couple decided to move into the penthouse of one building, at 35 Vestry St., court papers say.

At the time, Low claims, “many people” wanted to rent it, including a “professional athlete” who offered $30,000 a month, and a “successful professional recruiter” who offered “a similar amount with an option to purchase.”

But over Low’s objections, Robb allegedly put his wife in charge of renovations, turning the five bedrooms into three and “installing unique fixtures and implementing many personal specifications and upgrades” that cost at least $300,000.

Later, the couple decided they wanted the penthouse at the other site, 124 E. 13th St., with Webb “spending a significant amount of money to build out the space to suit their own tastes,” court papers say.

Low says the “protracted renovations” kept him from cutting a “lucrative” deal with “a famous actor” who “offered to rent the space . . . for two years for $360,000 per year.”

Low claims in his filing that after Robb and Webb separated, Robb moved into the East 13th Street penthouse in January 2009 and began scheming to cheat Low out of his 50 percent interest in the profits on both units and one occupied by Robb’s daughter, for which no one has ever paid rent to the development company.

Robb is also battling four of his seven siblings over claims he broke a promise to their ailing dad that he would manage his Wall Street business “to the benefit of family members” after he bought it from his father for $35 million in 1985.

Robb sold the firm for $180 million in 2001, but “did not make any meaningful distribution to his siblings of the proceeds of the family business,” court papers charge.